1. Field of the Invention
The present invention is directed generally to a process for forming thin-walled, elongated tubing members, particularly of steel or alloy steel, having superior mechanical properties. In particular, the invention is directed to a process for creating very high strength straight sidewall extruded high performance cartridge cases having superior mechanical properties.
2. Description of the Related Art
In today's high performance, high firing rate small and medium caliber guns and cannons, the firing chambers are relatively lightweight. This means that the cartridge case is not rigidly surrounded and contained in a massive structure so it is relatively free to expand and undergo some distortion when fired. This is especially true with regard to transverse dimensional integrity.
Therefore, in the manufacture of thin-walled, elongated, high strength tubing members, for use in high performance cartridge casings for medium caliber (20-50 mm) military applications, it is necessary to form the member from a material which will withstand the temperatures and extreme pressures associated with firing the shells. These include a sufficient fracture toughness to withstand the shock associated with firing and high strength such that when the shell expands during firing, it will thereafter contract predictably so that the ejection process can proceed normally and the shell will not stick in the firing chamber. This is especially critical in the case of straight sided firing chambers in which the shell is ejected by being caused to pass on through the chamber in a through-breech manner. Conventional medium caliber cartridges, on the other hand, employ tapered cartridge cases with tapered walls which exit the chamber through the same end as they enter. Dimensional tolerance and flow free case requirements for these conventional applications, while significant, are far less stringent than those for the straight sided case applications. Relatively large, thin walls characterize these cases which must fit a straight sided chamber, be fired, and then pass through the chamber to exit the system. Very little room for distortion exists if shell sticking is to be avoided.
High performance shell casings of the class described are manufactured from alloy steels, particularly high strength alloys. Particular materials which have been found very useful for such ammunition cases include modified American Iron and Steel Institute (A.I.S.I.) 4027 to 4042 grade which includes molybdenum and a small amount of chromium to insure proper hardenability. Materials which may later produce stringers or inclusions on the finished product must be eliminated or controlled. Boron grade steels, for example, are not recommended because titanium nitride inclusions may occur. These inclusions have the potential for allowing case splits or failures in the relatively long, thin-walled cases. Likewise, stringers in the finished product occasioned by the presence of uncontrolled residual alumina (Al.sub.2 O.sub.3) in the melt are undesirable for the same reason.
It is well known that iron and iron alloys may take one of several crystalline structures with respect to the position of the iron atoms in the structure. Austenite is one form defined as a solid solution of one or more elements in face-centered cubic iron. Although it may including other elements such as nickel and/or chromium, the solute is generally assumed to be carbon. Ferrite, on the other hand, is a solid solution of one or more elements in body-centered cubic iron, which, unless otherwise designated, is assumed to be carbon. Martensite, on the other hand, is defined as a metastable phase of steel formed by the transformation of austenite which occurs below an initial transition temperature known as the M.sub.s temperature. Martensite is an interstitial supersaturated solid solution of carbon and iron which has a body-centered tetragonal lattice. Its microstructure is characterized by an acicular or needle-like pattern.
Other structures encountered in heat treatment processes of interest to the process of the present invention include cementite, which is a compound of iron and carbon known chemically as iron carbide and having the approximate chemical formula Fe.sub.3 C. Cementite is characterized by an orthorhombic crystal structure and the chemical composition of a phase of the material may be affected by the presence of other carbide-forming elements such as manganese. Pearlite is a lamellar aggregate of ferrite and cementite.
Transformation from a face-centered structure such as austenite to a body-centered form such as martensite is normally accompanied by a volume expansion of the material. This is due to a rearrangement of the iron atoms to a structure that is less densely packed.
After one or more extrusion and ironing steps associated with conventional or prior art cartridge case manufacture, steel cases are heat treat hardened (quench and temper). This process creates a volume expansion and warpage characteristic totally unsuitable for the straight sided constant wall thickness case described by this disclosure.
Accordingly, it is a primary object of the present invention to provide a process for manufacturing very high strength, formed, cartridge cases of required transverse yield strength which meet necessary dimensional tolerance requirements.
It is a further object of the present invention to accomplish yield strength characteristics in alloy steel tubing utilizing a relatively inexpensive process which increases the yield strength in the transverse direction without warpage and produces relatively defect-free cartridge cases.